Handheld or otherwise portable analyzer devices are frequently used in the field e.g. for recognizing and sorting objects according to material(s) they contain. As a few examples, a portable analyzer device may be used in places like scrapyards, dumping grounds and recycling centers. While several techniques for analyzing a sample under study are available, optical emission spectroscopy is widely employed in portable analyzer devices to determine elemental composition of the sample under study. Such analyzer devices may be referred to as optical analyzers. An optical analyzer typically includes an excitation means for invoking an optical emission from a surface of a sample under study, a detector means for capturing signals that are descriptive of the optical emission and an analysis means for determination of the elemental composition of the sample under study on the basis of the captured signals.
A well-known example of such an optical analyzer employs laser-induced breakdown spectroscopy (LIBS) and may be referred to as a LIBS analyzer. A LIBS analyzer comprises, as the excitation means, a laser that is arranged to generate a high peak power laser pulse. The laser pulse is focused to the sample under study to form a plasma plume on a surface of the sample in order to cause atomization and excitation on the surface. This causes light emission at wavelength(s) that are characteristic to elements on the surface of the sample. The light emission is received at the detector means, which then carries out an analysis based on the received optical emission from the sample to determine the elemental composition of the sample. Since all elements emit light that exhibit wavelength(s) characteristic thereto, the relative intensities of different wavelengths in the light received at the detector means reveal the elemental constitution of the sample.
A dominant design of the detector means in a LIBS analyser, or in any optical analyser, makes use of a Czerny-Turner spectrometer known in the art. In a Czerny-Turner spectrometer the received light emission is transferred to an array detector via an optical path that involves one or more dispersing elements. FIG. 1 schematically illustrates an example that depicts operating principle of a Czerny-Turner spectrometer 100. The spectrometer 100 is shown with an entrance slit 102, a collimating mirror 104, a diffraction element 106, a focusing mirror 108 and an array detector 110. The diffraction element 106 may comprise e.g. a diffraction grating or a diffraction prism. In operation, the light entering via the entrance slit 102 hits the collimating mirror 104, from which collimated light is reflected to the diffraction element 106. The diffracted light from the diffraction element 106 hits the focusing mirror 108, from which the diffracted light is reflected to the array detector 110. The array detector 110 records signals that represent relative light intensities at different wavelengths, and these signals are provided for analysis to identify the element(s) that match the recorded relative light intensities. The route of the received light from the entrance slit 102 to the array detector 110 may be referred to as an optical path of the spectrometer 100.
Characteristics and relative positions of optical components of the spectrometer 100, i.e. the entrance slit 102, the collimating mirror 104, the diffraction element 106, the focusing mirror 108 and the array detector 110, define the range of wavelengths the spectrometer 100 is able to consider in the analysis. While such spectrometer can be applied for high-quality analysis, due to physical characteristics of the optical components of the spectrometer required to reach a sufficient range of wavelengths, the optical path defined by the optical components of the spectrometer 100 cannot be made arbitrarily short. In particular, the operation of the diffraction element 106 typically requires a certain minimum length for the optical path. In other words, the minimum size of the portable analyser employing the spectrometer 100 is limited due to the length of the optical path. On the other hand, having a portable analyser device of as small size as possible would be preferred to make the handling of the analyser device more convenient for the user and also to enable using the analyser device in narrow spaces when required.